(a) Field of the Invention
This invention relates to a signal detecting system in an optical information reading apparatus wherein, in order that a beam radiated from a light source is collected and is projected onto a recording medium having a track having recorded information and a light pencil modulated by the recorded information is received, four detectors divided respectively in the above mentioned track direction and the direction intersecting at right angles with it are arranged with the optical axis as a center in the far field of the above mentioned track.
(b) Description of the Prior Art
There is known, for example, such optical information reading apparatus wherein information is read by focusing a reading light spot through an objective on an information track recorded spirally or concentrically circularly on a recording medium. There is a recording medium having an information track and called, for example, a video disk, audio disk or data disk. In such disk, coded video signals, audio signals or data signals are recorded in the information track as optical information which can be represented by an optical transmission characteristic, reflection characteristic or phase characteristic. The information recorded in such disk is read by focusing through an objective on the information track a laser light radiated from a laser light source while the disk is rotated at a high speed and detecting a transmitted light or reflected light modulated by the information track. One of the features of such recording medium is that the information recording density is very high. Therefore, the width of each information track is very narrow and the spacing between the successive information tracks is also very narrow. In order to accurately read the original information from the information track thus narrow in both width and pitch, it is necessary to always accurately project on the track of the disk a beam spot focused by the objective. However, as the relative positions of the disk and objective fluctuate, the spot can not always be held on the track. Therefore, such optical reading apparatus is controlled by a servomechanism whereby the position lag of the beam spot from the information track is detected and the spot is displaced in the direction at right angles with the information track and the optical axis of the objective and the optical axis direction of the objective on the basis of this position lag signal.
FIG. 1 shows the optical system of the above described disk reading apparatus. A disk 1 is rotated at a speed, for example, of 1800 revolutions per minute by a spindle 2. A concentric circular or spiral track 3 is recorded on the disk 1. Such light as a laser light radiated out of a light source 4 is focused by a lens 5, half mirror 6, reflecting mirror 7 and objective 8 and is projected as a spot on the track 3 of the disk 1. The light reflected by the disk 1 is collected by the objective 8 and the light reflected by the reflecting mirror 7 and half mirror 6 is made to enter the light receiving device 10 through a lens 9. As shown in FIG. 2, this light receiving device has four detectors 11 to 14 divided respectively in the track direction (x--axis direction) and the direction (y--axis direction) intersecting at right angles with it. These detectors are arranged in the far field zone of the information track 3. That is to say, these detectors 11 to 14 are arranged at the position well away from the image of the pit structure formed by the objective 8 so that the diffracted beams of various orders formed by the pit structure of the information track can be detected as separated.
The following two methods of detecting tracking errors in such detecting system are considered.
The first is a method whereby, when the signals of the RF band detected by the respective detectors 11, 12, 13 and 14 are made respectively i.sub.1, i.sub.2, i.sub.3 and i.sub.4, the RF signal (i.sub.1 +i.sub.2 +i.sub.3 +i.sub.4) lagging by a 1/4 period phase will be superimposed on the signal of (i.sub.1 +i.sub.3)-(i.sub.2 +i.sub.4) and the signal obtained thereby will be passed through a low band passage filter to obtain a tracking error signal i.sub.TC. This method shall be explained more particularly. Generally, in a video disk, an information signal is recorded by being superimposed as FM modulated on a carrier of a predetermined frequency. It is known that, in such optical system as is shown in FIG. 1, in the case of reading the information signal recorded in the video disk, in case the optical axis of the objective 8 is on the track the beam entering the light receiving device 10 will be linearly symmetrical with respect to the track direction. However, in case the optical axis and track lag from each other, that is to say, in case a tracking error is present, the beam on the light receiving device 10 will be no longer linearly symmetrical with respect to the track direction. Therefore, in case there is no tracking error, (i.sub.1 +i.sub.3)-(i.sub.2 +i.sub.4) will become zero but, in case there is a tracking error, the signal of (i.sub.1 +i.sub.3)-(i.sub.2 +i.sub.4) will be obtained as a signal of the RF band the same as the video signal and the tracking error will be superimposed on it as an envelope. In order to obtain the tracking error signal i.sub.TC, it is necessary to extract only the envelope from this signal. The signal of (i.sub.1 +i.sub.2)-(i.sub.2 +i.sub.4) can be written as in the following formula: EQU (A+B sin 2.pi..nu.V.sub.t) sin .omega.t
wherein .nu. represents the frequency of the envelope (tracking error signal) and .omega. represents the frequency of the video signal (in the RF band). Further, .nu.&lt;&lt;.omega.. On the other hand, the video signal is in the form of cos .omega.t. Therefore, if the video signal (i.sub.1 +i.sub.2 +i.sub.3 +i.sub.4) as lagging in the phase by a 1/4 period (.pi./2) is superimposed on the signal i.sub.TC, aside from the coefficient, a signal of (A+B sin 2.pi..nu.V.sub.t) sin.sup.2 .omega.t will be obtained. If the signal obtained by this superimposition is applied to the low band passage filter, the sin.sup.2 .omega.t component of the RF band will be cut and only the envelope will be obtained. It will be obtained in the form of A+B sin 2.pi..nu.V.sub.t to be used as the signal i.sub.TC for detecting tracking errors.
Now the second method shall be described. It is a method whereby (i.sub.1 +i.sub.2)-(i.sub.3 +i.sub.4) is determined on the basis of the signals i.sub.1, i.sub.2, i.sub.3 and i.sub.4 in the tracking error signal band obtained from the respective detectors 11, 12, 13 and 14 of the light receiving device 10 and is used as a tracking error detecting signal i.sub.TP. This corresponds to dividing the detecting signal into two vertically (in the direction vertical to the track) and taking the difference between them. In this method, as different from the first method, the signal i.sub.TP will be obtained directly as a signal in the tracking error band (low frequency). In this case, too, the obtained signal will be in the form of A+B sin 2.pi..nu.V.sub.t wherein A and B depend on the spacial frequency of the pits and track spacing and the lag between the optical axis of the lens and the center of the light receiving device and A will be zero when the optical axis of the lens and the center of the light receiving device coincide with each other but will not be zero when they do not coincide with each other and will become larger in proportion to the amount .DELTA. of non-coincidence. Therefore, in such optical system wherein the track is traced by a lens moved independently of the light receiving device, when the lens is displaced following the eccentricity of the disk, such bias fluctuation signal A as has the disk rotation frequency as a basic frequency will be produced and will be a factor of a large error in the tracking signal detection.